144 ELEMENTS OF SCIENCE 



mutilated by having one of its solid angles removed, 

 such injury will be repaired by the deposition of fresh 

 material from the liquid. 



Crystals also possess the power of thus resuming growth 

 after interruption ; and there appears to be no limit to 

 the time after which the resumption of growth may 

 take place. A crystal may also undergo great internal 

 changes, and may be almost entirely disintegrated, yet 

 if a small portion remains, it may grow and perfect 

 itself again. Two crystals of different substances may 

 grow so as to become almost inextricably intermixed, 

 each of them preserving its individuality and growing 

 according to its own laws all the time. Crystals may 

 shoot out in an arborescent manner, as we often see in 

 the " frost " (which consists of crystals of ice) on a 

 window-pane. 



Stones, rocks, and other substances are said to be 

 " crystalline " when they are formed of minute crystals 

 aggregated together as is the case with marble and (as 

 before noted) with one state of sulphur. Other minerals 

 may be of similar chemical composition, but not formed 

 of minute crystals as, e.g., chalk, and one of the solid 

 states of sulphur.* The formation and growth of crys- 

 tals evidently takes place by most minute particles, 

 answering to the " molecules " spoken of in the last 

 chapter, each molecule being composed of " atoms," 

 according to the " atomic theory." It is supposed 

 that these atoms are persistent, indestructible, and indi- 

 visible, as well as unchangeable both in weight and 

 volume. It is also supposed that they are separated by 

 interstitial spaces, void save that they are occupied by 

 efcher. The increase or decrease of these spaces is thus 



* See ante, p. 141. 



